1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to frequency encoding, and more particularly, to frequency encoding and decoding which use parametric coding.
2. Description of the Related Art
Parametric coding is a method of expressing an audio signal with specific parameters. Parametric coding has been used in the MPEG-4 (Moving Picture Experts Group 4) standards.
In parametric coding, a sinusoidal component, a transient component, and noise are extracted from an audio signal, parameters thereof are determined, and a bitstream containing the parameters is generated.
If the sinusoidal component is extracted, sinusoidal tracking is performed in order to perform adaptive differential pulse code modulation (ADPCM) or DPCM on the sinusoidal component. In sinusoidal tracking, a sinusoidal signal successive to a sinusoidal signal in a previous or subsequent frame is detected from a current frame and then a correlation between the sinusoidal signals is determined.
A sinusoidal signal in a current frame that cannot be tracked from a sinusoidal signal in a previous frame is referred to as a birth sinusoidal signal or a birth partial. The birth sinusoidal signal is not successive to the sinusoidal signal in the previous frame but is newly generated in the current frame. Absolute coding must be performed on the birth sinusoidal signal instead of difference coding using the sinusoidal signal in the previous frame. For this reason, a large number of bits are needed for coding.
A sinusoidal signal in a current frame that can be tracked from a sinusoidal signal in a previous frame is referred to as a continuation sinusoidal signal or a continuation partial. Since difference coding that uses the sinusoidal signal in the corresponding previous frame can be performed on the continuation sinusoidal signal, coding can be effectively performed.
A sinusoidal signal that disappears without being successive to a sinusoidal signal in a subsequent frame from among continuation sinusoidal signals, is referred to as a death sinusoidal signal or a death partial.
Whether a sinusoidal signal is successive to another sinusoidal signal may be determined by checking whether the difference between the frequencies of the sinusoidal signals is less than or equal to a predetermined value. If the difference is less than or equal to the predetermined value, it is determined that there is continuation between the sinusoidal signals. Such a sinusoidal signal is determined to be a continuation sinusoidal signal and thus, difference coding is performed thereon.
FIG. 1 is a diagram illustrating a related art method of encoding the frequency of a continuation sinusoidal signal.
In the related art method, an audio signal is divided and encoded in units of sections, e.g., frames. Referring to FIG. 1, an audio signal is divided into three frames. The x-axis denotes time (t) and the y-axis denotes frequency (f). A sinusoidal signal in a frame has a representative frequency. Substantially, the frequency of the sinusoidal signal may change within the frame but it is assumed that for efficient encoding, the frequency of the sinusoidal signal does not change within the same frame.
Two birth sinusoidal signals 111 and 121 are present in a first frame. Since absolute coding is performed on the birth sinusoidal signals 111 and 121, the absolute frequencies of the birth sinusoidal signals 111 and 121 are encoded.
Two continuation sinusoidal signals 112 and 122 are present in a second frame. The continuation sinusoidal signals 112 and 122 are respectively encoded using the corresponding sinusoidal signals 111 and 121 in the previous frame. In detail, in the case of the continuation sinusoidal signal 112, the difference D1 between the frequencies of the continuation sinusoidal signal 112 and the corresponding previous sinusoidal signal 111 is encoded. Similarly, in the case of the continuation sinusoidal signal 122, the difference D3 between the frequencies of the continuation sinusoidal signal 122 and the corresponding previous sinusoidal signal 121 is encoded.
If the difference between representative frequencies of sinusoidal signals of the first and second frames is not large, it is possible to transmit the differences D1 and D3. However, if spectral components of sinusoidal signals in frames increase or decrease as illustrated in FIG. 1, the difference between representative frequencies of a sinusoidal signal in a current frame and a corresponding sinusoidal signal in a previous frame is large. In this case, if the frequency of the current sinusoidal signal is encoded using the difference between the frequencies of the current sinusoidal signal and the corresponding previous sinusoidal signal, a large number of bits are needed for the frequency encoding.